1. Field of the Invention
The present invention relates to an image forming apparatus that has a function which charges a photosensitive member surface using a charging roller. More particularly, the present invention relates to an image forming apparatus in which correction of a charging bias is possible.
2. Description of the Related Art
In recent years, a charging roller system that has a characteristic of suppressing ozone generation has been widely adopted as a charging mechanism of image forming apparatuses that use an electrophotographic method. For this charging roller, since a resistance value changes depending on the environment or life, a method has been proposed that determines an output bias based on a result obtained by detecting the charging current in order to apply the optimal bias in accordance with the change in resistance of the charging roller.
However, there is a problem that it is extremely difficult to accurately detect the charging current. The reason is that since, in particular, a current (charging current) in a charging roller in which the resistance value has increased changes accompanying the passage of time immediately after application of a bias (charging bias), the detection result will be different depending on the timing at which the current is detected. In the worst case an appropriate bias can not be output.
To solve this problem, for example, Japanese Patent Laid-Open No. 2004-205583 discloses a method which repeats detection of a current flowing in a charging member a plurality of times when applying a bias, and then starts an image forming operation when the variation amount from the time of the previous detection is lower than a certain threshold value. However, according to this method there is a problem that, in a case in which the resistance value of the charging roller increases to a large degree, time is required until the aforementioned variation amount becomes less than the threshold value, i.e. until the resistance value is stable, and thus the time until an image forming operation starts (so-called “aging time”) is extremely long. In contrast, in the latter half of the life of a charging roller, the relation between the charging current and the surface potential of the photosensitive drum changes from the relation in the first half of the life of the charging roller, and there is a problem that the bias cannot be properly corrected. This phenomenon can be explained as followed. That is, since the resistance value of a charging roller gradually increases together with the usage amount (life) thereof, it is necessary to increase the applied bias in accordance therewith. However, as the usage proceeds and the latter half of the life of the charging roller is entered, the bias value becomes a large value that exceeds a certain value and leakage current to the photosensitive drum starts to occur (however, this does not occur to a degree that imparts a physical defect to the photosensitive drum). If this situation occurs, even if the charging current flows well, the surface potential of the photosensitive drum itself does not rise very much. Therefore, even if the charging current is detected to perform bias correction, the required surface potential can not be obtained.